Resilient return device for rotatable shafts in electric switching equipment and the like



7, 67 R. c. DURAND ETAL RESILIENT RETURN DEVICE FOR ROTATABLE SHAFTS IN ELECTRIC SWITCHING EQUIPMENT AND THE LIKE Filed Dec. 15, 1964 5 Sheets-Sheet 1 s N Q 7 R. c. DURAND ETAL 3,298,244

RESILIENT RETURN DEVICE FOR ROTATABLE SHAFTS IN ELECTRIC SWITCHING EQUIPMENT AND THE LIKE Filed Dec. 15, 1964 s Sheets-$heet z Jan. 17, 1967 R. c. DURAND ETAL 3, 98,244 -RESILIENT RETURN DEVICE FOR ROTATABLE SHAFTS IN ELECTRIC SWITCHING EQUIPMENT AND THE LIKE Filed Dec.

5 Sheets-Sheet 3 United States Patent f 3 298 244 RESILIENT RETURN DEi lCE FUR ROTATABLE SHAFTS TN ELECTREC SWHTJHHNG EQUIPMENT AND THE LIKE Roger C. Durand, Saint-Germain-en-Laye, and lean G.

La Fosse, Mareil-Marly, France, assignors to La Telemecanique Electrique, Nanterre, France, a company of France Filed Dec. 15, 196 1, Ser. No. 418,500 Claims priority, application France, Dec. 23, 1963, 958,233 12 Claims. (Cl. 74-504) This invention relates to an improved spring device for resiliently restraining a rotatable shaft in a prescribed neutral position and returning the shaft to that position when rotated in either direction away from the neutral position.

The device is especially useful in connection with electrical switching equipment such as multi-positional rotary controllers, and will be described with especial reference to this last application.

A known type of multi-positional controller for manually controlling complex electrical apparatus comprises a camming drum mounted on a shaft for rotation by means of a manual lever or handle. The drum has a plurality of camming projections spaced axially and/or circumferentially of its periphery, the projections being desirably arranged to be selectively presettable at prescribed positions on the drum. A set of microswitches, connected in the circuitry of the electrical apparatus to be controlled, is arranged adjacent the drum, e.g. in a linear array parallel to a generatrix of the drum, so as to be actuated during rotation of the drum on engagement of a camming projection of the drum with an actuator element of a microswitch.

In such a rotary controller, there is one angular position of the drum which is a neutral position wherein, for example, all of the microswit-chcs may be deactivated and the associated apparatus deenergized. Resilient means are generally provided for restraining the drum in this neutral position and returning it thereto on release of the control lever after the lever has been displaced in one or the other direction away from the neutral position.

The resilient restraining and two-way returning means heretofore used for this purpose have generally not been entirely satisfactory. Thus, in one known such arrangement, there is provided a pre-tensioned helical torsion spring coaxial with the shaft and having outturned end legs engaging stop means on the stationary frame structure, and the shaft has a driver pin projecting from it and interposed between the legs of the springs so as to deform the spring in torsion when the shaft is displaced in either direction away from neutral.

A serious defect of such an arrangement is that the action of the spring obeys Hookes law, so that the restoring couple acting on the shaft increases proportionally with the angular displacement of the shaft from neutral. Since the range of angular displacement may well attain 180 in each direction, the restoring force will tend to be inadequate for small displacements and excessive for large ones. This defect is even more objectionable in cases where spring detent means are provided for producing a moderate resilient resistance to rotation as the shaft is moved past each of one or more intermediate 3,Z%,Zd4 Patented Jan. 17, 1967 positions, as is frequently the case in controller apparatus of the type contemplated.

Objects of this invention include the provision of a resilent return device of the type specified in which the spring restoring couple can be made approximately uniform over a wide range of angular displacements; in which the shaft will be restored to neutral under a controlled, and if desired substantially uniform, velocity without tendency to overshoot or rebound; intermediate detent positions, if any are provided, will be positively and reliably traversed; and to provide such a device which will be simple and sturdy and inexpensive and especially well-suited for use in multi-positional rotary controllers and analogous switching equipment.

An exemplary embodiment of the invention will now be described with reference to the accompanying drawings, wherein:

FIGURE 1 is a side view of an electrical controller apparatus embodying the invention.

FIGURE 2 is a corresponding view in overhead plan.

FIGURE 3a is an end view as viewed in IIIIII, FIGURE 1, showing the resilient return device of the invention in neutral position.

FIGURES 3b and 3c are similar views showing the device in its two end positions respectively, and

FIGURE 4 is an exploded view of the device.

The rotary cont-roller switch assembly generally illustrated in FIGURES 1 and 2 comprises a supporting frame in the form of a pair of end plates 4 and 5 and four spacer rods or posts 3 having ends engaging suitable holes formed in the corners of the end plates and retained in assembly therewith by means of screw 12.

A shaft, not visible in the drawings except for its extreme end portion 19, is arranged for rotation in coaxial openings formed in the end plates 4 and 5 :and is formed with an extension 2 beyond end plate 4 in the form of a manual lever crank or handle for rotating the shaft.

Mounted on the shaft between end plates 4, 5 is a controller drum generally designated T and formed in this embodiment of the invention as a stack of identical spool-like elements 1 all assembled in juxtaposed relation along the shaft 19 for bodily rotation with said shaft when the handle 2 is turned one way or the other. The spoollike elements 1 are desirably made of a tough plastic material and each element comprises a cylindrical body and a pair of disk-like side flanges coaxial with the body and of larger diameter than it, as will be apparent from FIGURE 1. The inwardly directed surface of each of the side flanges of each spool-like element ll has a series of oval bosses 6 projecting from it towards the other side flange.

Cylindrical camming pins 7 are adapted to be selectively inserted between pairs of adjacent bosses 6 so as to be removably retained between said bosses and the cylindrical surface of the body of the spool.

An array of identical microswitc'hes 8 is supported from the end frame members 4, 5 of the assembly as by way of the flanged strips shown. Each microswitch 8, of generally conventional construction, has an actuator element 8:: projecting from it. The actuator elements 8a of all the microswitches are aligned substantially parallel to a common generatrix of the drum T so as to be selectively depressible by the pins '7 of the respective drum elements 1, by way of intermediate levers or the like partly shown at 8b, when the drum T is rotated with handle 2.

In FIGURES 1 and 2 the controller switch assembly is illustrated in a neutral position, in which a pointer 2a associated with handle 2 is positioned in registry with a zero calibration of a suitable dial 9 associated with the assembly, this being the lowermost position of pointer 2a in the drawings. From this neutral position the handle 2 can be turned in either direction. It will be understood that the microswitches 8 are connected with the circuitry, not shown, of electrical apparatus to be controlled by means of the controller switch assembly. The camming pins 7 are preset at selected positions on the respective spool-like drum elements 1, in accordance with a prescribed programme. Actuation of the handle 2 will then serve to operate the electrical apparatus in accordance with the programmed sequence and with current operational requirements.

The controller switch assembly so far described is generally conventional and does not, per se, form a part of this invention. Improved means will now be described in accordance with the invention for resiliently returning the rotatable assembly, including the drum T, the shaft and handle, which spring return means is generally designated 119, to its neutral position when displaced therefrom in either sense by means of the handle 2.

The spring return means 119 is mounted on the outer side of the end frame plate remote from the handle 2, and its component parts are best seen in the exploded view of FIGURE 4. This assembly includes a flanged supporting frame plate or stirrup 11 having a central aperture, diametrically-opposed side legs 23 turned down at right angles to the plane of the plate 11, and outturned apertured securing lugs 11a. The lugs 11a are secured to the end plate 5 of the assembly frame by means of the screws 12 engaging threaded holes formed in the adjacent ends of the spacer bars 3, and thus contribute to holding the entire structure in assembly.

The rotatable shaft carrying the drum T has an end fitting secured on its projecting end beyond the end plate, said fitting being shown at the extreme right in FIGURE 4. This end fitting includes a flange 19b seated against the outer surface of end plate 5, a cylindrical hub 19 having a pair of square driver pins or lugs 20 projecting from diametrically opposed points of it, and a smaller diameter end hub or nose 1%.

A generally octogonal plate 13B is centrally recessed to fit around the hub portion 19 of the shaft. The recess in plate 13b includes as shown a pair of diametrically opposed arcs 14 forming part of a common circumference and each somewhat less than 90 in extent, and a pair of larger-radius sector-shaped cutout portions 15 interconnecting said arcs. The dimensioning of the various portions of the recess is such that the plate can be fitted around the hub 19, with the two opposite, smaller-radius arcs 14 of the recess rotatably engaging the cylindrical surface of said hub, and the driver pins projecting from said hub then riding in the sector cutouts 15 of the recess so as to limit the rotation of the plate 13B relative to the shaft through abutment of the pins 20' with radial sidewalls of the sector cutouts.

Over plate 13B is seated a plastic friction washer 16, and over this washer is positioned a further plate 13A exactly similar to plate 13B just described. The outer surface of plate 13a is seated in frictional engagement with the inner surface of flanged frame plate 11, as indicated in FIGURE 1.

The plates 13a and 131) each have a pair of opposite lugs 17 projecting from their opposite ends and bent at right angles to the plate. A pair of extension coil springs 18 have their hooked ends anchored to lugs 17 of the respective plates, each spring having its one end hooked around a lug 17 projecting from plate 13A and its other end hooked around a lug projecting from plate 13B.

Each of the plates 13A and 1313 has edge portions 21 which are inclined at an angle of substantially 45 to the 4 main axis of the plate as defined by a line joining the spring-anchoring lugs 17 thereof.

Considering then the actions of the tension springs 18, it will be seen that each spring tends to rotate both plates 13A, 1313 in opposite directions until the rotation of both plates is arrested through abutment of inclined edges 21 of the plates with a common inner surface of a related one of the two downturned flanges 23 of the frame cover plate 21. Due to the above defined angular relationship of said edges 21, it will be apparent that in this abutted position the two plates 13A and 13B are arrested in a mutually orthogonal relationship, in which corresponding radial directions of the respective plates are at right angles to each other, as shown in FIGURE 3a.

The sector cutout portions 15 of the recess in each plate 13A, 13B is somewhat greater than in arcuate extent. More precisely, the arcuate extent of each sector portion 15 may be made substantially equal to 90 plus the width of a driver lug 20. Hence in the mutually orthogonal abutted position of the two plates 13A, 13B just described, each sector cutout 15 of one plate overlaps a related sector cutout 15 of the other plate an amount corresponding to the width of a driver lug 20, with the overlapping portions of the two pairs of sector cutouts 15 being diametrically opposed. Consequently the abutted position into which the plates 13A, 13B tend to be rotated by the joint action of the extension springs 18, as shown in FIGURE 3a, serves to define a unique angular position for the rotatable assembly of the structure relative to the frame thereof. This unique position is taken as the neutral position above referred to, in which the pointer 2a lies opposite a zero mark on dial 9.

When the handle 2 is turned in either sense to displace said movable assembly away from the neutral position as just defined, the extension springs 18 exert a restoring couple tending to return the assembly 19 to neutral, and the value of the restoring couple can be seen to be twice the product of the tension of a spring 18 times the distance (designated A in FIGURE 3a of the centre of rotation of the assembly from the centre line of action of either spring 18.

Considering this spring restoring action in greater detail, it is seen that when handle 2 is rotated in one sense, the clockwise sense as seen from the right of FIGURE 1, so that the movable assembly is rotated in the counterclockwise direction indicated by arrow F1 in FIGURE 3b, plate 13A is rotated in the said direction bodily with assembly 10 through the action of both driver lugs 20 upon radial sides of the sector cutouts 15 of said plate 13A, rotation of plate 13A in this direction being permitted owing to the fact that its inclined edges 21 are then moved away from the stationary flange surfaces 23 against which said edges were abutted. On the other hand, plate 13B remains in its abutted position because, firstly, driver lugs 20 are moving freely within the sector cutouts 15 of said plate 13B without acting on the radial sides thereof, and, secondly, rotation of plate 13B in the direction F1 is positively prevented through the abutment of the inclined edges 21 of the plate against the stationary flange surfaces 23.

Similarly but conversely, when handle 2 is turned to rotate the assembly in the direction shown by arrow F2 in FIGURE 30, plate 13B is rotated bodily with the assembly whereas plate 13A remains stationary.

During displacement of the rotatable assembly in either direction from its neutral position, the tension of both springs 18 increases substantially proportionally with displacement in accordance with the normal law of action of springs of this type. However it will be apparent that at the same time as the tension of each spring increases, the distance from the centre line of each spring to the centre line of rotation (the distance called A in FIGURE 3a) simultaneously decreases. Hence the restoring couple, which as mentioned above is the product of the two quantities (tension and distance) does not increase in proportion to displacement as it would tend to do if a conventional spring restoring device were used rather than the spring-restoring device of the invention herein described. Such an increase of the restoring couple proportional to displacement as would be obtained with a conventional spring arrangement is highly objectionable in that it requires the operator to exert an ever increasing muscular force on the handle such as 2 as the handle is turned increasing angles in either direction from neutral.

With the spring restoring arrangement described, on the contrary, the restoring couple remains approximately constant as the angle of rotation is increased from zero, over a substantial angular range to either side of the neutral position.

More precisely, it can be shown that the spring restoring couple which opposes displacement of the rotatable assembly by means of handle 2 to either side from neutral, is substantially proportional to a cosine function of the displacement angle. In accordance with the well-known properties of the cosine function, there fore, the couple remains constant, to a second order of approximation, for small displacement angles and thereafter falls off toward zero as the displacement angle approaches 90. In fact, it is evident that when the lever has been turned by about 90 from its neutral position, as to either of the positions shown in FIGURES 3b and 3c, the distance termed A in FIGURE 3a would approach zero. Hence the restoring force would tend to drop to unacceptably low values for large rotation angles.

This condition is obviated in accordance with the invention in that the nose fixture of the shaft includes a projecting cylindrical section 1% beyond the hub portion 19, so that the extension springs 18 are forced into contact with the peripheral surface of the projecting section 19a when the angular displacement of the assembly exceeds a predeterminable amount, whereupon the springs are deflected as shown in FIGURES 3b and 3c and the distance previously referred as A is positively prevented from approaching zero but is made to retain the constant value determined by the radius R of the projecting section 19a. It can be shown that with this arrangement, the restoring couple of the springs actually increases somewhat as the rotation is continued beyond the angle at which the springs 18 first contact the periphery of cylindrical portion 19a.

In this manner it is possible readily to arrange matters so that the over-all variation in the spring-restoring couple over the full range of angular displacement of the rotatable assembly 10 is relatively very small, and said couple may, for practical purposes, be considered as virtually constant.

When the handle 2 is released after maximum rotation thereof to the right or left, the rotatable assembly including the drum T and the plate 13A or 13B that was displaced therewith, returns to its neutral position under the approximately constant force exerted by the spring arrangement described. Due to this constant restoring force, and also to the frictional resistance developed between the particular plate ]l3A or 138 which is moving, with the washer 16 on one side and with the outer surface of flange 1% or the inner surface of frame plate 11, as the case may be, on the other side, the rotatable assembly is returned to neutral position at a moderate and substantially uniform velocity and so has no tendency to overshoot the neutral position or rebound therefrom.

In the rotary controller switch device illustrated, detent means are prvoided for resiliently arresting or retarding the rotation of the drum T at each of a number of prescribed angular positions in order to make the operator aware that such a position has been reached in the manipulation of handle 2. Such detent means are generally designated 24 in FIGURE 1 and include levers supported from the upper spacer posts 3 and resiliently cooperating with the previously mentioned camming rods 7 inserted around the periphery of the drum T. The detent means 6 form the subject matter of co-pending patent application Serial No. 418,400, now Patent No. 3,242,762, for Detent Device for a Rotatable Member in Electrical Switching Equipment and the Like and hence need not be here described in greater detail.

It will be seen that the invention has provided a highly advantageous two-way spring-restoring arrangement for a shaft rotatable in opposite directions from a neutral position, especially for use in a rotary switching device such as a multipositional rotary controller for electrical equipment. The arrangement has made it possible to achieve a substantially uniform spring-restoring couple over the full extent of the angular displacement of the shaft in either direction, whereby said uniform couple can be predetermined at an optimum value such that it will reliably ensure a positive return of the rotatable assembly at a moderate velocity without overshoot or rebound and will cause said assembly to move past any detent means (such as the means 24) if such are provided without imposing an excessive muscular effort on the operator at any point over the range of the angular displacement. The improved device may be found useful in various contexts, including cases Where the rotatable shaft to be resiliently restrained is rotated mechanically (e.g. in limit switches) rather than manually as herein shown.

Various modifications may 'be made in the single emtment shown and described while remaining within the terms of the claims. Thus, while the essential components of the device, including spring 18, sector recesses 15 and lugs 20 are provided in duplicate in the construction shown, it is clear that an operative device according to the invention may readily be constructed in which such components are single.

It should also be observed that the various cooperating components of the device, including especially the driver pins 20, plates 13A and 13B, and frame lugs 23, as well as the springs 13, may be designed in a variety of forms other than those shown and described above in detail. Essentially, the desired two-way restoring effect is obtained through the provision of the following entities: a first and a second elements mounted for indepenent rotation around the shaft (i.e. herein the first and second plates 13A, 13B); first cooperating stop means on the first element and the shaft engagea-ble to limit rotation of the first element relative to the shaft in a first direction (i.e., one radial sidewall of each cutout 15 of plate 13A and a related side of each lug 20); second cooperating stop means on the second element and the shaft engageable to limit rotation of the second element relative to the shaft in the opposite direction (i.e., the opposite radial sidewall of each cutout 15 of plate 13B .and a related opposite side of each lug 20); third cooperating stop means on the first element and the frame engageable to limit rotation of the first element relative to the frame in said first direction (i.e., an edge 21 of plate 13A engageable with the inner surface of a leg 23); fourth cooperating stop means on the second element and the frame engageable to limit rotation of the second element relative to the frame in said opposite direction (i.e., an edge 21 of plate 13B engageable with the inner surface of a leg 23); and spring means 18 interconnecting both elements to tend to rotate the first element 13A in said first direction and rotate the second element 13B in said opposite direction. Such an arrangement will ensure that said elements 13A, 13B will resiliently restrain the shaft in, and return it to, a prescribed neutral position relative to the frame.

What we claim is:

1. In rotary electric switching apparatus, a resilient return device for a switching shaft rotatable in opposite directions from a neutral position relative to a shaftsupporting frame, comprising:

a first and a second elements mounted for independent rotation around the shaft;

first cooperating stop means on the first element and the shaft engageable to limit rotation of the first element relative to the shaft in a first direction;

second cooperating stop means on the second element and the shaft engageable to limit rotation of the second element relative to the shaft in the opposite direction;

third coperating stop means on the first element and the frame engageable to limit rotation of the first element relative to the frame in said first direction;

fourth cooperating stop means on the second element and the frame engageable to limit rotation of the second element relative to the frame in said opposite direction; and

spring means interconnecting both elements so as to tend to rotate the first element in said first direction and rotate the second element in said second direction; whereby said elements will resiliently restrain said shaft in, and return the shaft to a prescribed neutral position relative to the frame.

2. The invention claimed in claim 1, wherein said spring means comprises an extension spring having opposite ends attached to the respective elements, said spring having its line of action lying substantially in a plane transverse to the geometric axis of said shaft at a radial distance therefrom.

3. The invention claimed in claim 2, including means defining a cylindrical surface coaxial with the shaft and of a radius less than the value of said radial distance when the shaft is in neutral position, and engageable by said extension spring during shaft rotation away from said neutral position to deflect the spring and prevent the line of action thereof from approaching the shaft axis to within less than a predetermined minimal radial distance.

4. In rotary electric switching apparatus, a resilient return device for a switching shaft rotatable in opposite directions from a neutral position relative to a shaftsupporting frame, comprising:

a driver pin projecting from the shaft;

' a first plate mounted for rotation around the shaft and having a cutout therein with a radial wall of said cutout engageable with one side of said pin to limit the rotation of said first plate relative to the shaft in a first direction;

a second plate mounted for rotation around the shaft and having a cutout therein with a radial wall of said cutout engageable with the opposite side of said pin to limit the rotation of said second plate relative to the shaft in the opposite direction;

a stop surface on the first plate engageable with a cooperating stop surface on the frame to limit rotation of the first plate relative to the frame in said one direction;

a stop surface on the second plate engageable with a cooperating stop surface on the frame to limit rotattion of the second plate relative to the frame in said opposite direction; and

spring means connected to both plates to tend to rotate the first plate in said first direction and rotate the second plate in said opposite direction;

whereby said plates will resiliently restrain said shaft in and return the shaft to a prescribed neutral posi tion relative to the frame.

5. The invention claimed in claim 4, wherein said spring means comprises an extension spring having opposite ends attached to the respective plates, said spring having its line of action substantially in a plane transverse to the geometric axis of said shaft at a radial distance therefrom.

6. The invention claimed in claim 5, including means defining a cylindrical surface coaxial with the shaft and of a radius less than the value of said radial distance when the shaft is in neutral position and engageable by said extension spring during shaft rotation away from said neutral position to deflect the spring and prevent its line of action from approaching the shaft axis to within less than a predetermined minimal radial distance.

7. In rotary electric switching apparatus, a resilient return device for a switching shaft rotatable in opposite directions from a neutral position relative to a shaftsupporting frame, comprising:

two diametrically opposed driver pins projecting from the shaft;

a first plate mounted for rotation around the shaft and having two diametrically opposed cutouts therein With a radial wall of each cutout engageable with one side of a related one of said pins to jointly limit the rotation of said first plate relative to the shaft in a first direction;

a second plate mounted for rotation around the shaft and having two diametrically opposed cutouts therein with a radial wall of each cutout engageable with the opposite side of a related one of said pins to jointly limit the rotation of said second plate relative to the shaft in the opposite direction;

stop surface means on each of said first and second plates engageable with cooperating stop surface means on the frame to limit rotation of said first and second plates relative to the frame respectively in said first and said opposite directions; and

two extension springs each having its opposite ends attached to points of the respective plates, said springs having their lines of action lying substantially in a common plane transverse to the shaft at equal radial distances from the geometric axis of said shaft, so as to tend to rotate the first plate in said first direction and rotate the second plate in said opposite direction;

whereby said plates will resiliently restrain said shaft in and return the shaft to a prescribed neutral position relative to the shaft.

8. The invention claimed in claim 7, including means on said shaft defining a coaxial cylindrical surface of a radius less than the common value of the radial distance of each spring when the shaft is in said neutral position, and engageable by said extension springs during shaft rotation away from the neutral position to deflect the springs and prevent their lines of action from approaching the shaft axis to within less than a predetermined minimal radial distance.

9. The invention claimed in claim 7, wherein said stop surface means on each of said plates comprises edge surfaces of the respective plates inclined at opposite angles from the line of symmetry of the pair of cutouts formed in the respective plates, and said cooperating stop means comprises a common planar surface projecting from the frame.

10. The invention claimed in claim 9, wherein said frame includes a supporting surface transverse to the shaft and having an aperture therein through which the shaft projects, and including a flanged stirrup-like frame member having a portion transverse to the shaft axially spaced from said supporting surface and two diametrically opposed fianged leg portions secured to said supporting surface, each of said leg portions defining said common planar surface on its inwardly directed side, and said plates are rotatably supported on said shaft in axially adjacent relation and are retained between said supporting surface and said transverse portion of the frame member, each plate having two opposite parallel edge surfaces to define said stop surface means cooperating with the respective planar surfaces defined on the inwardly directed side of a respective leg portion.

11. The invention claimed in claim 10, including hub structure on the shaft in the part thereof projecting through said aperture, said hulb structure including a first cylindrical bearing surface for rotatably receiving said plates thereon and having said pins projecting there- 9 10 from and a second cylindrical surface coaxially extend- 12. The invention claimed in claim 7, including fricing said first bearing surface, said second surface being tion means braking the rotation of said plates.

of a radius less than the value of said equal radial distances when the shaft is in said neutral position and en- References Cited by the Examiner gageable by said extension springs during shaft rotation 5 UNITED STATES PATENTS away from the neutral position to deflect the springs and 2,642,502 6/1953 Johnsom prevent their lines of action from approaching the shaft 2 954 707 10 19 0 Kalous 7 5 axis to within less than a prescribed minimum radial distance. MILTON KAUFMAN, Primary Examiner. 

1. IN ROTARY ELECTRIC SWITCHING APPARATUS, A RESILIENT RETURN DEVICE FOR A SWITCHING SHAFT ROTATABLE IN OPPOSITE DIRECTIONS FROM A NEUTRAL POSITION RELATIVE TO A SHAFTSUPPORTING FRAME, COMPRISING: A FIRST AND A SECOND ELEMENTS MOUNTED FOR INDEPENDENT ROTATION AROUND THE SHAFT; FIRST COOPERATING STOP MEANS ON THE FIRST ELEMENT AND THE SHAFT ENGAGEABLE TO LIMIT ROTATION OF THE FIRST ELEMENT RELATIVE TO THE SHAFT IN A FIRST DIRECTION; SECOND COOPERATING STOP MEANS ON THE SECOND ELEMENT AND THE SHAFT ENGAGEABLE TO LIMIT ROTATION OF THE SECOND ELEMENT RELATIVE TO THE SHAFT IN THE OPPOSITE DIRECTION; THIRD COPERATING STOP MEANS ON THE FIRST ELEMENT AND THE FRAME ENGAGEABLE TO LIMIT ROTATION OF THE FIRST ELEMENT RELATIVE TO THE FRAME IN SAID FIRST DIRECTION; FOURTH COOPERATING STOP MEANS ON THE SECOND ELEMENT AND THE FRAME ENGAGEABLE TO LIMIT ROTATION OF THE SECOND ELEMENT RELATIVE TO THE FRAME IN SAID OPPOSITE DIRECTION; AND SPRING MEANS INTERCONNECTING BOTH ELEMENTS SO AS TO TEND TO ROTATE THE FIRST ELEMENT IN SAID FIRST DIRECTION AND ROTATE THE SECOND ELEMENT IN SAID SECOND DIRECTION; WHEREBY SAID ELEMENTS WILL RESILIENTLY RESTRAIN SAID SHAFT IN, AND RETURN THE SHAFT TO A PRESCRIBED NEUTRAL POSITION RELATIVE TO THE FRAME. 